1. Field of the Invention
The present invention relates to wireless communication systems, and, in particular, to orthogonal frequency division multiple access (OFDMA) transmission.
2. Description of the Related Art
The 3rd Generation Partnership Project (3GPP) is a collaboration agreement established in December 1998 to bring together a number of telecommunications standards bodies, known as “Organizational Partners,” that currently include ARIB, CCSA, ETSI, ATIS, TTA, and TTC. The establishment of 3GPP was formalized in December 1998 by the signing of the “The 3rd Generation Partnership Project Agreement”.
3GPP provides globally applicable standards as Technical Specifications and Technical Reports for a 3rd Generation Mobile System based on evolved GSM core networks and radio access technologies that they support (e.g., Universal Terrestrial Radio Access (UTRA) for both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes). 3GPP also provides standards for maintenance and development of the Global System for Mobile communication (GSM) as Technical Specifications and Technical Reports including evolved radio access technologies (e.g., General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE)). Technical Specifications for current standards related to mobile telephony are generally available to the public from the 3GPP organization.
3GPP is currently studying the evolution of the 3G Mobile System and considers contributions (views and proposals) directed toward the evolution of the UTRA Network (UTRAN). A set of high-level requirements was identified by 3GPP workshops including: reduced cost per bit; increased service provisioning (i.e., more services at lower cost with better quality); flexibility of use of existing and new frequency bands; simplified architecture with open interfaces; and reduced/reasonable terminal power consumption. A study on the UTRA & UTRAN Long Term Evolution (UTRAN-LTE, also known as 3GPP-LTE and E-UTRA) was started in December 2004 with the objective to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. The study considered modifications to the radio-interface physical layer (downlink and uplink) such as means to support flexible transmission bandwidth up to 20 MHz, introduction of new transmission schemes, and advanced multi-antenna technologies.
3GPP-LTE is based on a radio-interface incorporating orthogonal frequency division multiplex (OFDM) techniques. OFDM is a digital multi-carrier modulation format that uses a large number of closely-spaced orthogonal sub-carriers to carry respective user data channels. Each sub-carrier is modulated with a conventional modulation scheme, such as quadrature amplitude modulation (QAM), at a (relatively) low symbol rate when compared to the radio frequency (RF) transmission rate. In practice, OFDM signals are generated using the fast Fourier transform (FFT) algorithm.
Consequently, in a 3GPP-LTE transmitter, user data is error encoded, mapped into a symbol constellation, reference pilot signals added, and a serial-to-parallel conversion applied to group the multiplexed symbols/reference pilots into sets of tones (in the frequency domain). An N-point inverse fast Fourier transform (IFFT) is applied to each set, where the integer size, N, of the N-point IFFT depends on the number of OFDM channels. The output of the IFFT is a set of complex time-domain samples. A parallel-to-serial conversion is applied to this time-domain sample stream before conversion from the digital domain to analog domain by a digital-to-analog converter (DAC). The DAC is clocked at the FFT sampling rate of the IFFT. The analog signal is then modulated and transmitted through the wireless medium.
One aspect of OFDM systems is that a number of low-rate streams are transmitted in parallel instead of a single high-rate stream, since low symbol rate modulation schemes (i.e., where the symbols are relatively long compared to the channel time characteristics) exhibit less inter-symbol interference (ISI) from multipath conditions. Since the duration of each symbol is long, a guard interval is inserted between the OFDM symbols to eliminate ISI. A cyclic prefix (CP) is transmitted during the guard interval, which consists of the end of the OFDM symbol copied into the guard interval. The OFDM symbol follows the guard interval. The guard interval includes of a copy of the end of the OFDM symbol so that the receiver can integrate over an integer number of sinusoid cycles for each of the multipath signals demodulating the OFDM signal with an FFT. Spectral efficiency (i.e., the ratio of useful OFDM symbol length to the total OFDM symbol length) increases with a shorter CP. Although the guard interval contains redundant data, reducing the capacity of some OFDM systems, a long guard interval allows transmitters to be spaced farther apart in a single-frequency network (SFN), and longer guard intervals allow larger SFN cell-sizes or better coverage in mountainous regions where signal delay spread is relatively large.
FIG. 1 shows a prior art table of values for a current numerology for the FFT specifications of an OFDM transmit data path architecture as proposed by 3GPP-LTE in 3GPP TR 25.814 v7.0.0 (2006-06). A 3GPP-LTE transmitter operates with one or more of six transmission bandwidths (BWs): 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. In each OFDM sub-frame, the sub-frame duration is 0.5 ms with 15 kHz sub-carrier spacing and a useful sub-frame period of 66.67 ms. 3GPP-LTE specifies a short CP and a long CP: if a short CP is used, a sub-frame comprises 7 OFDM symbols, while, if a long CP is used, a sub-frame comprises 6 OFDM symbols. As the transmission BW increases, the corresponding FFT size and FFT sampling frequency also increases.
In FIG. 1, the FFT sampling rates are oversampled by 1.7×, whereby the oversampling ratio is defined as the ratio of the FFT sampling rate to the occupied RF bandwidth. Over-sampling of FIG. 1 provides frequency-domain filtering to reject adjacent channel interference (ACI) in the frequency domain, and obviates a requirement to reject ACI in the time domain (i.e., after the IFFT at the 3GPP-LTE transmitter and before the FFT at a 3GPP-LTE receiver). This high oversampling rate results from the constraint that the FFT sampling rate needs to be an integer multiple of 3.84 MHz (e.g., 2×, 4×, 6×, and 8×3.84 MHz). The constraint provides a simple rule for generating the various clock rates employed by the proposed 3GPP-LTE transmit data path architecture, but compromises power efficiency of a particular FFT implementation. The power efficiency is defined as the ratio of the occupied RF bandwidth to the FFT sampling rate In the current numerology, the power efficiency of the OFDM symbol is between 59.3% (best case) and 58.6% (worst case), which implies that between 40.7% and 41.4% of the power consumed by a processor performing the FFT (or IFFT) is wasted. Wasted processing power results in shorter battery life for a mobile handset (e.g., the user equipment (UE)).